 True believers desire to truly live holy. When you become born again and the Holy Spirit comes to live in you, he begins to give you a new nature. He gives you new desires. So though you may make mistakes, though you may slip up from time to time, a true believer will always have that end goal in mind, to be more like Jesus. Not perfection, but progress. As long as you are being perfected, you're under the process of sanctification and therefore a true born again believer.